This paper presents a new directional coupler design, which can increase power capacity working with S band. The design concept is based on multi-stage coupled structure. Aim is to increase the size of coupling aperture by reducing the coupling degree of each stage structure. In view of this, multi-stage coupled structure theory is utilized to improve the directivity of directional coupler, coupling flatness and power capacity. According to the derivation of theory, it can be deduced that the sum of the coupling degree of single stage coupling structure is equal to the coupling degree of two-stage coupling directional coupler (TSCDC), and the directivity of TSCDC depends on the directivity of the first stage coupling structure. Then, rectangular waveguide two-stage coupled directional coupler is designed and fabricated. The measured results demonstrate full-band from 1.72 to 2.61 GHz with coupling flatness < 0.65 dB and the directivity > 26 dB.
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